2022ApJ...933..224F

The BL Lac object PKS 0735+178 has shown some complex multiwavelength variation phenomena in previous studies, especially in its color behavior. Bluer-when-brighter, redder-when-brighter, and achromatic behavior were all found to be possible long-term trends of PKS 0735+178. In this work, we collected long-term multiwavelength data on PKS 0735+178, and also performed a multicolor optical monitoring on intraday timescales. Intraday variability was detected on one night. On long timescales, a possible 22 day time lag was found between the R and γ-ray bands. The results of a cross-correlation analysis exhibited strong correlations between various optical bands on both intraday and long timescales. However, only a mild correlation was found between the long-term γ-ray and R-band light curves, which could be interpreted as different emission mechanisms for the γ-ray and optical emissions. PKS 0735+178 showed a significant harder-when-brighter (HWB) behavior in the γ-ray band, which is consistent with the observed optical bluer-when-brighter (BWB) trend on both long-term and intraday timescales. We found that the HWB and BWB trends will be enhanced during active states, especially for the historical low state. Such a phenomenon indicates a special activity-dependent color behavior of PKS 0735+178, and it could be well interpreted by the jet emission model.